BACKGROUND
[0001] This invention relates to the copolymerization of a mono-1-olefin monomer, such as
ethylene, with a higher alpha-olefin comonomer.
[0002] It is well known that mono-1-olefins, such as ethylene, can be polymerized with catalyst
systems employing vanadium, chromium or other metals on supports such as alumina,
silica, aluminophosphate, titania, zirconia, magnesia and other refractory metals.
Initially, such catalyst systems were used primarily to form homopolymers of ethylene.
It soon developed, however, comonomers such as propylene, 1-butene, 1-hexene or other
higher, mono-1-olefins were copolymerized with ethylene to provide resins tailored
to specific end uses. Often, high density and/or high molecular weight copolymers
can be used for blow molding applications and the blow molding process enables rapid
processing into a desired molded product. Unfortunately, these copolymers often are
plagued by various types of surface roughness as a result of a constant desire to
increase processing rates.
[0003] This surface roughness has been described loosely in the past as having "melt fracture
instabilities" or "worms". Worms, or melt fracture instabilities, can be defined broadly
as irregularities and instabilities, such as anomalous, ridge-like structures, that
are formed during melt processing and are clearly observed on the inside of an otherwise
smooth, blow molded article. Worms can occur randomly and intermittently on either
the interior or exterior surface of the molded article and can detach from the surface,
causing unacceptable contamination of the contents or even structural degradation
of the molded article. Generally, melt fracture instabilities are observed only on
the interior of the molded article because the heat of the die, or mold, can cause
smoothing of the exterior surface of the molded article.
[0004] Variance of the shear rates (extruder screw RPMs) for each type of copolymer can
affect the melt fracture instabilities. At low shear rates, the extrudate usually
is smooth and exhibits no melt fracture instabilities. As shear rates are increased,
the extrudate can have a matte, or sharkskin-type, finish which is characterized by
fine scale irregularities on the extrudate surface. At even higher shear rates, slip-stick,
spurt, or cyclic melt fracture can be observed. At the slip-stick point, the pressure
in the extruder periodically oscillates between high and low pressure. Worms are formed
and can always be seen at the slip-stick point of an extrusion process, herein defined
as the critical shear rate. Finally, as screw speed in increased even further, the
copolymer can enter a period of continuous slip. Another way to describe critical
shear rate is the overall velocity over the cross section of a channel in which molten
polymer layers are gliding along each other or along the wall in laminar flow.
[0005] Most polymer processing operations occur within a limited window of extrusion (shear),
or production, rates. Obviously, one way to avoid melt fracture instabilities is to
limit, i.e., decrease, production rates and use very low extrusion rates. Thus, an
improved polymer is one which either does not exhibit melt fracture instabilities
at higher shear rates, i.e. has a higher critical shear rate. However, while it is
possible to increase the critical shear rate by increasing polymer melt index and/or
decreasing polymer molecular weight distribution, other polymer properties will be
negatively affected. Therefore, it is very desirable to produce a polymer that does
not encounter melt fracture instability, i.e., a polymer that has high critical shear
rates. Furthermore, increasing polymer production rates into articles of manufacture
while minimizing melt fracture instabilities is an efficient use of polymer product
and processing equipment.
SUMMARY OF THE INVENTION
[0006] Therefore, it is an object of this invention to provide an improved olefin polymerization
process.
[0007] It is another object of this invention to provide a process to produce copolymers
of ethylene and mono-1-olefins that can be processed at increased production rates
and have increased critical shear rates.
[0008] It is still another object of this invention to provide a process to produce copolymers
of ethylene and mono-1-olefins that have a broadened melt processing window.
[0009] It is yet another object of this invention to provide a process to produce copolymers
of ethylene and mono-1-olefins that have increased critical shear rates without the
loss of other polymer physical properties.
[0010] It is still another object of this invention to provide a composition comprising
copolymers of ethylene and mono-1-olefins having higher critical shear rates that
can be processed at high production rates into articles of manufacture.
[0011] In accordance with this invention, herein is provided a polymerization process comprising
contacting:
a) ethylene monomer;
b) at least one mono-1-olefin comonomer having from about 2 to about 8 carbon atoms
per molecule;
c) a catalyst system comprising chromium supported on a silica-titania support, wherein
said support comprises less than about 5 weight percent titanium, based on the weight
of the support, and wherein said catalyst system has been activated at a temperature
within a range of about 900°F to about 1050°F; and
d) a trialkyl boron compound,
wherein said contacting occurs in a reaction zone in the absence of hydrogen, at
a temperature within a range of about 180°F to about 215°F,
and recovering an ethylene/mono-1-olefin copolymer.
[0012] In accordance with another embodiment of this invention, a copolymer comprising ethylene
and a mono-1-olefin having from about 3 to about 8 carbon atoms per molecule is provided,
wherein said copolymer has a density within a range of about 0.935g/cc to about 0.96g/cc;
a high load melt index (HLMI) within a range of about 0.5 g/10 minutes to about 30
g/10 minutes; and a critical shear rate for the onset of slip-stick melt fracture
of greater or equal to about 1000 sec
-1.
[0013] In accordance with this invention, there is provided a polymerization process consisting
essentially of contacting:
a) ethylene monomer;
b) at least one mono-1-olefin comonomer having from about 2 to about 8 carbon atoms
per molecule;
c) a catalyst system comprising chromium supported on a silica-titania support, wherein
said support comprises less than about 5 weight percent titanium, based on the weight
of the support, and wherein said catalyst system has been activated at a temperature
within a range of about 900°F to about 1050°F; and
d) a trialkyl boron compound,
wherein said contacting occurs in a reaction zone in the absence of hydrogen, at
a temperature within a range of about 180°F to about 215°F,
and recovering an ethylene copolymer.
[0014] In accordance with another embodiment of this invention, a copolymer consisting essentially
of ethylene and a mono-1-olefin having from about 3 to about 8 carbon atoms per molecule
is provided, wherein said copolymer has a density within a range of about 0.935g/cc
to about 0.96g/cc; a high load melt index (HLMI) within a range of about 0.5 g/10
minutes to about 30 g/10 minutes; and a critical shear rate for the onset of slip-stick
melt fracture of greater or equal to about 1000 sec
-1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] The terms "polymer" and "copolymer" are used interchangeably in this disclosure.
Both terms include a polymer product resulting from polymerizing ethylene monomer
and a mono-1-olefin, or higher alpha-olefin, comonomer, selected from the group consisting
of propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, and/or 4-methyl-1-pentene.
Catalyst Systems
[0016] As used in this disclosure, the term "support" refers to a carrier for another catalytic
component. However, by no means, is a support necessarily an inert material; it is
possible that a support can contribute to catalytic activity and selectivity.
[0017] The catalyst system support used in this invention must be a silica-titania support.
As used in this disclosure, references to "silica" mean a silica-containing material
generally composed of 80 to 100 weight percent silica, the remainder, if any, being
selected from one or more inorganic oxides, as disclosed in the art, useful as catalyst
system supports. For instance, a silica-containing material can consist essentially
of silica and no more than 0.2 weight percent of alumina or other metal oxides. Other
ingredients which do not adversely affect the catalyst system or which are present
to produce some unrelated result can also be present. The support must contain less
than about 5 weight percent titanium (Ti), based on the weight of the support. Preferably,
the support comprises from 2 to about 5, most preferably 2 to 4, weight percent titanium,
in order to produce a polymer with the most desirable physical properties.
[0018] Silica-titania supports are well known in the art and can be produced as disclosed
in Dietz, U.S. Patent No. 3,887,494, the disclosure of which is hereby incorporated
by reference.
[0019] The catalyst component must be a chromium compound. The chromium compound, or component,
can be combined with the silica-titania support in any manner known in the art, such
as by forming a coprecipitated tergel of the silica, titanium, and chromium components.
Alternatively, an aqueous solution of a water soluble chromium component can be added
to a hydrogel of the silica-titania component. Suitable water soluble chromium compounds
include, but are not limited to, chromium nitrate, chromium acetate, and chromium
trioxide. Alternatively, a solution of a hydrocarbon soluble chromium component, such
as tertiary butyl chromate, a diarene chromium compound, biscyclopentadienyl chromium(II)
or chromium acetylacetonate, can be used to impregnate the silica-titania xerogel
which results after removal of water from the cogel.
[0020] The chromium component is used in an amount sufficient to give about 0.05 to about
5, preferably 0.5 to 2 weight percent chromium, based on the total weight of the chromium
and support after activation.
[0021] The resulting chromium component on a silica-titania support then is subjected to
activation in an oxygen-containing ambient in any manner conventionally used in the
art. Because of economy, the preferred oxygen-containing ambient is air, preferably
dry air. Activation can be carried out at an elevated temperature for about one-half
to about 50 hours, preferably for about 2 to about 10 hours, at a temperature within
a range of about 900°F to about 1050°F (about 455°C to about 565°C), preferably from
about 965°F to about 1020°F (about 520° to about 550°C). Under these calcination conditions
at least a substantial portion of any chromium in a lower valence state is converted
to the hexavalent form.
[0022] After calcination or activation, the oxidized, supported catalyst system is cooled
to about room temperature, e.g. about 25°C, under an inert atmosphere, such as argon
or nitrogen. The catalyst system must be kept away from contact with reducing compounds,
water, or other detrimental, or deactivating, compounds until use. The catalyst system
used in the inventive process must not be subjected to a reduction treatment. A reduction
treatment can cause narrowing of the molecular weight distribution (MWD). This MWD
narrowing can increase the critical shear rate for the onset of melt fracture instabilities
during polymer processing and can result in surface roughness of the extruded article
of manufacture.
[0023] A cocatalyst must be used in conjunction with the catalyst system; the cocatalyst
must be a trialkyl boron compound, wherein the alkyl group has from about 1 to about
12 carbon atoms, preferably about 2 to about 5 carbon atoms per alkyl group. Exemplary
trialkyl boron compounds include, but are not limited to, tri-n-butyl borane, tripropylborane
and triethylborane (TEB). These cocatalysts can be effective agents to improve resultant
polymer properties, such as, for example, reducing melt flow and retarding polymer
swelling during polymerization. By far, the most preferred cocatalyst is triethylboron
(TEB), due to ease of use in the polymerization reactor and best improvement of polymer
properties.
[0024] The trialkyl boron cocatalyst can be used in an amount within a range of about 1
to about 20 parts per million (ppm), or milligrams per kilogram (mg/kg), based on
the mass of ethylene monomer in the reactor. Preferably, the cocatalyst is used in
an amount within a range of about 2 to about 10 ppm, and most preferably, within a
range of about 3 to about 6 ppm, for cost effectiveness and best polymer properties.
[0025] Optionally, the trialkyl boron cocatalyst can be used in conjunction with a small
amount of trialkyl aluminum cocatalysts. While not wishing to be bound by theory,
it is believed that a small amount of a trialkyl aluminum cocatalyst can be used as
a preservative for the trialkyl boron cocatalyst, to protect the trialkyl boron cocatalyst
from inadvertent contact with air, or oxygen.
[0026] Exemplary trialkyl aluminum cocatalysts include, but are not limited to, triethylaluminum,
ethylaluminum sesquichloride, diethylaluminum chloride, and mixtures thereof. Preferably
the trialkyl aluminum cocatalyst is triethyl aluminum for best catalyst system and
trialkyl boron cocatalyst compatibility.
[0027] The trialkyl aluminum cocatalyst, if used, can be used in an amount within a range
of about 0.1 to about 5 parts per million (ppm), or milligrams per kilogram (mg/kg),
based on the mass of diluent in the reactor. Preferably, the trialkyl aluminum cocatalyst
is used in an amount within a range of about 0.5 to about 3 ppm, and most preferably,
within a range of about 0.5 to about 2 ppm, for cost effectiveness and best polymer
properties.
Reactants
[0028] Polymers produced according to the process of this invention must be copolymers.
This inventive process is of particular applicability in producing copolymers of ethylene
and higher alpha-olefins. Ethylene monomer must be polymerized with a comonomer selected
from the group consisting of propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 4-methyl-1-pentene,
and mixtures thereof. Ethylene is the most preferred monomer, due to the advantageous
properties of the resultant copolymer. Preferably, the comonomer is 1-hexene and/or
4-methyl-1-pentene to achieve maximum polymer toughness.
[0029] Comonomer is added to the polymerization reactor, or reaction zone, in an amount
within a range of about 0.5 to about 15 weight percent, preferably within a range
of about 1 to about 10 weight percent, based on the weight of monomer. Most preferably,
the comonomer is present in the reaction zone within a range of about 2 to about 6
weight percent in order to produce a polymer with the most desired properties, such
as, for example, reduced melt fracture instabilities. Another method to express the
amount of comonomer is to specify the amount measured in the reactor flash gas. Generally,
the amount of comonomer present in the reactor flash gas is in an amount within a
range of about 0.05 to about 6 mole percent, based on the reactor diluent, such as
isobutane. Preferably, the cocatalyst is present in the flash gas in an amount within
a range of about 0.1 to about 2 mole percent, and most preferably, within a range
of about 0.3 to about 1 mole percent, for cost effectiveness and best polymer properties.
Polymerization
[0030] Polymerization of ethylene and the comonomer must be carried out under slurry, or
particle form, polymerization reaction conditions wherein the reactor temperature
is kept below the temperature at which polymer goes into solution. Such polymerization
techniques are well known in the art and are disclosed, for instance, in Norwood,
U.S. Patent No. 3,248,179, the disclosure of which is hereby incorporated by reference.
[0031] The temperature of the polymerization reactor, or reaction zone, according to this
invention, is critical and must be kept within a range of about 180°F to about 215°F
(about 82°C to about 102°C), preferably within a range of about 180°F to about 195°F
(about 82 to about 90°C). Most preferably, the reaction zone temperature is within
a range of 180°F to 185°F (82°C to 85°C). Although higher reactor temperatures can
be used, operating outside of the specified temperature ranges can produce a copolymer
which can be more subject to swelling during polymerization or higher melt fracture
instabilities.
[0032] The slurry process generally is carried out in an inert diluent (medium), such as,
for example, a paraffin, cycloparaffin, and/or aromatic hydrocarbon. Exemplary diluents
include, but are not limited to propane, n-butane, isobutane, n-pentane, 2-methylbutane
(isopentane), and mixtures thereof. Isobutane is the preferred diluent due to low
cost and ease of use.
[0033] Pressures in the slurry polymerization process can vary from about 110 to about 700
psia (0.76-4.8 MPa) or higher. The catalyst system is kept in suspension and can be
contacted with the monomer and comonomer(s) at sufficient pressure to maintain the
medium and at least a portion of the monomer and comonomer(s) in the liquid phase.
The medium and temperature are thus selected such that copolymer is produced as solid
particles and copolymer is recovered in that form. Catalyst system concentrations
in the reactor can be such that the catalyst system content ranges from 0.001 to about
1 weight percent based on the weight of the total reactor contents.
[0034] Two preferred polymerization methods for the slurry process are those employing a
loop reactor of the type disclosed in Norwood and those utilizing a plurality of stirred
reactors either in series, parallel or combinations thereof wherein the reaction conditions
are different in the different reactors. For instance, in a series of reactors a chromium
catalyst system can be utilized either before or after a reactor utilizing a different
catalyst system. In another instance, a chromium catalyst system can be utilized in
parallel with another reactor employing a polymerization different catalyst system
and the resulting polymerization products can be combined prior to recovering a copolymer.
[0035] In accordance with this invention, hydrogen cannot be present in the polymerization
reactor during polymerization. The presence of hydrogen results in a decrease and
lowering of the critical shear rate for the onset of melt fracture for the resultant
polymer product. While not wishing to be bound by theory, it is believed that the
absence of hydrogen can leave a high molecular weight tail on the polymer which results
in a broader molecular weight distribution. Polymers having a broader molecular weight
distribution can have less melt fracture, i.e., a higher critical shear rate.
[0036] The catalyst system, cocatalyst, monomer, and comonomer can be added to the reaction
zone in any order, according to any method known in the art. For example, the catalyst
system, cocatalyst, monomer, and comonomer can be added simultaneously to the reaction
zone. If desired, the catalyst system and cocatalyst can be precontacted under an
inert ambient prior to contacting the monomer and/or comonomer.
[0037] Optionally, precontacting of the catalyst system and cocatalyst prior to the catalyst
system contacting ethylene can reduce the amount of cocatalyst necessary in the reaction
zone. This precontacting can reduce the amount of trialkyl boron cocatalyst necessary
in the reactor zone by up to a factor of ten (10).
Product
[0038] Polymers produced in accordance with this invention are a copolymer of ethylene and
at least one higher mono-1-olefin comonomer. Copolymers produced according to this
invention have a broad molecular weight distribution and therefore have higher critical
shear rates and reduced melt fracture instabilities. Additionally, the production
rate of these copolymers into articles of manufacture can be significantly increased;
consequently, copolymers produced according to this invention exhibit higher production
rates during blow molding processes.
[0039] The density of these novel copolymers usually is within a range of about 0.935g/cc
to about 0.96, preferably from about 0.94 to about 0.958g/cc. Most preferably, the
copolymer density is within a range of about 0.945 to about 0.955g/cc.
[0040] Another defining physical characteristic of these copolymers is the high load melt
index (HLMI). Usually, the HLMI is within a range of about 0.5 to about 30 g/10 minutes,
preferably within a range of about 3 to about 10 g/10 minutes. Most preferably, the
HLMI is within a range of about 4 to about 8 g/10 minutes.
[0041] Copolymers produced according to this invention also have very high critical shear
rates. Generally, the critical shear rate for the onset of melt fracture of these
novel polymers is greater than or equal to about 1000 sec
-1, preferably greater than or equal to about 1500 sec
-1. Most preferably, the critical shear rate of polymers produced in accordance with
this invention is within a range of about 1800 sec
-1 to about 6000 sec
-1.
[0042] A further understanding of the present invention and its advantages are provided
by reference to the following examples.
EXAMPLES
[0043] Ethylene-hexene copolymers were prepared in a continuous particle form process by
contacting the catalyst with the monomers, employing a liquid full loop reactor, having
a volume of 23 gallons (87 liters), isobutane as the diluent, and occasionally some
hydrogen, as shown in the Examples. The reactor was operated to have a residence time
of 1.25 hrs. The reactor temperature was varied over the range of 180°C to 215°C,
unless stated differently, and the pressure was 4 MPa (580 psi). At steady state conditions,
the isobutane feed rate was 46 1/hr, the ethylene feed rate was about 30 lbs/hr, and
the 1-hexene feed rate was varied to control the density of the product polymer. Polymer
was removed from the reactor at the rate of 25 lbs/hr. The catalyst systems used were
commercially available catalyst systems purchased from W.R. Grace and Company, the
Davison business unit, designated as 963 Magnapore®.
[0044] Polymer product was collected from each run and tested according to the following
procedures:
Density (g/ml): ASTM D 1505-68 and ASTM D 1928, Condition C. Determined on a compression
molded sample, cooled at about 15°C per minute, and conditioned at room temperature
for about 40 hours.
High Load Melt Index (HLMI)(g/10 min): ASTM D 1238, condition E. Determined at 190°C
with a 21,600 gram weight.
Heterogeneity Index (HI): Mw/Mn
Critical Shear Rate (Onset of Worms) (sec-1): The determination of Critical Shear Rate was developed by Dr. Ashish Sukhadia for
Phillips Petroleum Company as a result of a need to accurately determine the onset
of worms. The testing apparatus is an Extruder Capillary Set-Up which consists of
a one (1) inch Killion® (KL-100) single screw extruder that is used to provide a pressurized
polymer melt to a die through a connecting adaptor. Each die consists of two separate
pieces: 1) an entry die (zero land length) and 2) a land die (a die having the same
diameter as the entry die but with a land, constant diameter, region). A complete
determination results in a flow curve for the tested material. The procedure consists
of extruding the material (polymer) first with the entry die alone. The flow rate,
pressure drop in the extruder, pressure drop in the adaptor (mounter just prior to
the dies) and melt temperature are recorded. Then, a land die of desired land length
is fitted at the end of the entry die and the experimental procedure is repeated.
In addition to the data, the visual appearance of the extrudate (strand) is recorded
for both the entry die and entry die plus land die experiments. The data are used
to calculate the apparent shear rate and shear stress. Standard calculation methods
are used; see C.D. Han, Rheology on Polymer Processing, pp 89-126, Academic Press, NY (1976). In addition, a graphic plot of the flow curve
(true shear stress vs. apparent shear stress) is plotted. The following calculations
are used:

where
- L
- = Land length of capillary, inch
- D
- = Diameter of capillary die, inch = Diameter of land die, inch
- Q
- = volumetric Flow Rate, inch3//sec
- 'Υapp
- = Apparent Shear Rate, 1/sec
- τapp
- = Apparent Shear Stress, MPa
- τtrue
- = True (Corrected) Shear Stress, MPa
- ΔPent
- = Entrance Pressure Drop, MPa = Pressure Drop through orifice die
- ΔP
- = Total Pressure Drop, MPa = Pressure Drop through orifice + land die
[0045] The following dies and conditions were used:
Entry die diameter: 0.080 inch, 90° cone entry angle
Land die: 0.080 inch diameter, 2.25 inch land length (L/D ratio = 15)
Temperature: 215°C flat temperature profile for extruder and capillary
Example 1
[0046] Polymer samples were prepared as described above. Different catalyst system activation
and different levels of triethylboron (TEB) were used. Triethylaluminum (TEA) was
not added to the reactor. Runs 105 and 106 are commercially available polymers, used
for comparison.
TABLE 1
Run |
Catalyst Activation, °C |
Reactor Temp, °F |
TEB, mg/kg |
Density, g/cc |
HLMI, g/10 mins |
HI, (Mw/Mn) |
Critical Shear Rate, Sec-1 |
101 |
1100 |
200 |
0 |
0.953 |
5.2 |
34 |
359 |
102 |
1100 |
200 |
2 |
0.953 |
6.4 |
57 |
349 |
103 |
1000 |
194 |
2.1 |
0.954 |
9.5 |
49 |
>2200 |
104 |
1000 |
195 |
1.1 |
0.949 |
9.4 |
47 |
>2200 |
105(a) |
N/A |
N/A |
N/A |
0.955 |
6.7 |
32 |
487 |
106(b) |
N/A |
N/A |
N/A |
0.956 |
5.4 |
32 |
484 |
(a) Commercially available polyethylene from Mobile, HYA. |
(b) Commercially available polyethylene from Novacor. |
N/A = Not available. |
[0047] The data in Table 1 show that TEB can be used in conjunction with a chromium catalyst
system to reduce melt fracture. Comparison of Run 101 with 102 and Run 103 with 104
shows that higher levels of TEB desirably delay the onset of melt fracture, or worms,
by allowing higher (faster) extruder screw speeds. The data in Table 1 further demonstrate
that a lower catalyst activation temperature also can raise the onset of worms.
Example 2
[0048] Polymer samples were prepared as described above. Different catalyst system activation
temperatures and different levels of triethylboron (TEB) were used. Triethylaluminum
(TEA) was not added to the reactor.
TABLE 2
Run |
Activation Temperature (°F) |
Hydrogen H2/C2= (mole ratio) |
TEB (ppm in i-C4=) |
Critical Shear Rate (Onset of Worms) (1/sec) |
201 |
1100 |
0 |
5.9 |
1800 |
202 |
1100 |
0.2 |
6.2 |
1080 |
203 |
1000 |
0 |
5.3 |
>2200 |
204 |
1000 |
0 |
5.4 |
>2200 |
205 |
1000 |
0.144 |
5.8 |
1650 |
[0049] The data in Table 2, again, demonstrate that lower catalyst system activation temperatures
can postpone the onset of worms, until higher extruder speeds. The data in Table 2
also show that the absence of hydrogen in the polymerization reactor allows higher
extruder throughputs, or screw speeds, before the onset of worms.
[0050] While this invention has been described in detail for the purpose of illustration,
it is not to be construed as limited thereby but is intended to cover all changes
and modifications within the spirit and scope thereof.
1. A polymerization process comprising contacting at a temperature within a range of
82 - 102 °C in the absence of hydrogen:
a) ethylene monomer;
b) at least one mono-1-olefin comonomer having from 2 to 8 carbon atoms per molecule;
c) a catalyst system comprising chromium supported on a silica-titania support, wherein
said support comprises from 2 to 10 weight percent titanium, based on the weight of
the support, and wherein said catalyst system has been activated at a temperature
within a range of 482 - 566 °C; and
d) a trialkyl boron compound;
and recovering an ethylene/mono-1-olefin copolymer.
2. The process of claim 1 wherein said temperature is within a range of 82 - 91 °C.
3. The process of claim 1 or 2 wherein said mono-1-olefin comonomer is selected from
propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 4-methyl-1-pentene, and mixtures
thereof.
4. The process of any of the preceding claims wherein said catalyst system has been activated
at a temperature within a range of 518 - 549 °C.
5. The process of any of the preceding claims wherein said trialkyl boron compound is
selected from tri-n-butyl borane, tripropylborane, triethylborane, and mixtures thereof.
6. The process of claim 5 wherein said trialkyl boron compound is triethylborane.
7. The process of any of the preceding claims wherein said comonomer is 1-hexene.
8. The process of claim 1 wherein said ethylene/mono-1-olefin copolymer has
a) a density within a range of 0.935 to 0.96 g/ml
b) a high load melt index within a range of 0.5 to 30 g/10 min; and
c) a critical shear rate for the onset of melt fracture of about 1200 sec-1 or above.
9. An ethylene/higher mono-1-olefin copolymer having:
a) a density within a range of 0.935 to 0.96 g/ml
b) a high load melt index within a range of 0.5 to 30 g/10 min; and
c) a critical shear rate for the onset of melt fracture of about 1200 sec-1 or above.
10. The copolymer of claim 9 having a density within a range of 0.940 to 0.955 g/ml.
11. The copolymer of claim 9 having a high load melt index within a range of 1 to 20 g/10
min.
12. The copolymer of claim 9 having a critical shear rate for the onset of melt fracture
of greater than 1900 sec-1.
13. A composition comprising a copolymer as defined in any of the claims 9 - 12.